time-selector: implemented time selection with spin buttons
time-selector: removed unused var
time-selector: ref builder objects before destroying it

Fixes the annoying bug
time-select: hide period selector when not 12h format
time-selector: limit to 11 hours when using 12h format
time-selector: select AM by default
time-selector: format hour according to the 12/24h setting
time-selector: format time label when the day period changes

Also, factor out date formatting code.
time-selector: always show the popover at the bottom
time-selector: update label when hour or minute changes
time-selector: send GcalTimeSelector::modified signal when date changes
edit-dialog: remove useless code

I'm not really sure of this one, but it seems very correct. At least, things work now.
edit-dialog: properly set time selector's sensivity

Instead of simply not settings them at all, it is now set according to the writable and all day values.
time-selector: removed unneeded code